Study of Multiferroic Properties of Ferroelectric

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MSU Graduate Theses

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Study of Multiferroic Properties of Ferroelectric- Ferromagnetic Study of Multiferroic Properties of Ferroelectric- Ferromagnetic

Heterostructures BZT-BCT/LSMO Heterostructures BZT-BCT/LSMO

Md Abdullah-Al Mamun Missouri State University, [email protected]

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STUDY OF MULTIFERROIC PROPERTIES OF FERROELECTRIC-

FERROMAGNETIC HETEROSTRUCTURES BZT-BCT/LSMO

A Masters Thesis

Presented to

The Graduate College of

Missouri State University

TEMPLATE

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science, Materials Science

By

Md Abdullah-Al Mamun

August 2017

ii

Copyright 2017 by Md Abdullah-Al Mamun

iii



Physics, Astronomy, and Materials Science

Missouri State University, August 2017

Master of Science


ABSTRACT

Currently, there has been a flurry of research focused on multiferroic materials due to

their potential applications. Lead (Pb)-based ferroelectric and multiferroic materials

(PZT, PMN-PT, PZN-PT etc.) have been widely used for sensors, actuators, and electro-

mechanical applications due to their excellent dielectric and piezoelectric properties.

However, these materials are facing global restriction due to the toxicity of Pb. In this

thesis, multiferroic properties of ferroelectric-ferromagnetic heterostructures consist of

Pb-free perovskite oxides 0.5Ba(Zr0.2Ti0.8)O3-0.5 (Ba0.7 Ca0.3)TiO3 (BZT-BCT) and

La0.7Sr0.3MnO3 (LSMO) have been studied. The heterostructures BZT-BCT/LSMO were

fabricated on LaAlO3 (LAO) and Pt substrates by pulsed laser deposition. Structural and

crystalline qualities of the films have been investigated through theta-2theta scan, rocking

curve, and phi-scan of X-Ray diffraction (XRD) and Raman spectroscopy. Ferroelectric

and ferromagnetic properties have been characterized using the Sawyer-Tower method, a

SQUID magnetometer, and Ferromagnetic resonance (FMR) spectroscopy. A well-

behaved magnetization-magnetic field (M-H) hysteresis has been observed in LSMO as

well as heterostructures, indicating ferromagnetism in the films. FMR spectroscopy data

support the static magnetization data obtained using SQUID. These results may guide the

development of next-generation lead-free ferroelectric-ferromagnetic heterostructures for

magnetoelectric device applications.

KEYWORDS: epitaxy, polarization, piezoresponse, double-exchange, Gilbert damping.

This abstract is approved as to form and content

_______________________________

Kartik Ghosh, PhD

Chairperson, Advisory Committee

Missouri State University

iv



By


A Masters Thesis

Submitted to the Graduate College

Of Missouri State University

In Partial Fulfillment of the Requirements

For the Degree of Master of Science, Materials Science

August 2017

Approved:

_______________________________________

Kartik Ghosh, PhD

_______________________________________

Mahua Biswas, PhD

_______________________________________

Keiichi Yoshimatsu, PhD

_______________________________________

Julie Masterson, PhD: Dean, Graduate College

In the interest of academic freedom and the principle of free speech, approval of this thesis indicates the

format is acceptable and meets the academic criteria for the discipline as determined by the faculty that

constitute the thesis committee. The content and views expressed in this thesis are those of the student-

scholar and are not endorsed by Missouri State University, its Graduate College, or its employees.

v

ACKNOWLEDGEMENTS

I would like to express my gratitude to my advisor, Dr. Kartik Ghosh, for the

supervision, encouragement, and advice throughout my master’s years. He has cared

every step of my progress in the thesis. I would also like to thank Dr. Robert Mayanovic

for his cooperation performing different experiments.

I would like to thank the department of Physic, Astronomy, and Materials Science

for providing excellent lab facilities to complete my experiments. I am also thankful to

the graduate students of our department to assist me different ways. I want to

acknowledge Air Force Research Laboratory (AFRL) at WPAFB, Dayton OH for their

cooperation to do some experiments there through my adviser.

I am thankful to my beloved wife Mohsina Jannat who keeps patience in time of

difficulties of my research. She encouraged me at tough situations. Finally, I would like

to thank my parents for their constant support.

vi

TABLE OF CONTENTS

Overview ..............................................................................................................................1

Fabrication and characterization of Ferroelectric (FE)-Ferromagnetic (FM)

heterostructures BZT-BCT/LSMO/LAO using pulsed laser deposition .............................6

Abstract ....................................................................................................................6

Introduction ..............................................................................................................7

Experimental ............................................................................................................8

Results and Discussion ..........................................................................................10

Conclusions ............................................................................................................24

Acknowledgement .................................................................................................25

References ..............................................................................................................25

Ferromagnetic resonance study of BZT-BCT/LSMO heterostructure grown on

different substrates-LAO and Platinum using Pulsed Laser Deposition ...........................29

Abstract ..................................................................................................................29

Introduction ............................................................................................................30

Experimental ..........................................................................................................32

Results and Discussion ..........................................................................................34

Conclusions ............................................................................................................50

References ..............................................................................................................51

Conclusions ........................................................................................................................53

References ..........................................................................................................................54

vii

LIST OF TABLES

Table 1. Calculated out-of-plane lattice parameter from XRD with Standard values

from Literature (all the values in Angstrom) .....................................................................12

Table 2. Peak positions and FWHM of the corresponding vibrational modes observed

in Raman spectroscopy ......................................................................................................16

Table 3. Remnant and saturation polarization with coercive field at different voltages

applied to FECAP ..............................................................................................................17

Table 4. Results from SQUID measurement .....................................................................22


in Raman spectroscopy of BZT-BCT/LSMO thin films on Pt and LAO substrates .........40

Table 6. Static Magnetic properties derived from SQUID measurement for all the

samples ...............................................................................................................................42

Table 7. Dynamic magnetic properties of all the samples calculated from FMR..............49

viii

LIST OF FIGURES

Figure 1. X-ray diffraction pattern of both LSMO (red line) and BZT-BCT/LSMO

(black line) on LAO substrate. The inset plot shows the splitting of (002) peak, which

is a characteristic of the tetragonal crystal structure ..........................................................11

Figure 2. High resolution rocking curve profiles for the (a) BZT-BCT (002) (b) BZT-

BCT (003), and (c) LSMO (002) reflections. ....................................................................12

Figure 3. Raman spectra of BZT-BCT/LSMO thin film on LAO substrate with

Gaussian and Lorentzian peak fitting by using Origin software. .......................................13

Figure 4. Polarization-electric field (P-E) hysteresis loops for ferroelectric capacitors

Au-Ti/BZT-BCT/LSMO at different applied voltages ......................................................16

Figure 5. Frequency dependence of dielectric constant (a) and dielectric loss (b) for

BZT–BCT/LSMO thin film on LAO substrate. .................................................................18

Figure 6. Leakage current characteristics of Au-Ti/BZT-BCT/LSMO with two different

applied electric fields 100 kV/cm (black line) and 400 kV/cm (red line). ........................20

Figure 7. (a) Magnetization-Applied field (M-H) hysteresis loop of LSMO/LAO (red),

BZT-BCT/LSMO/LAO (purple) obtained at room temperature and (b) the low field

fragments of the hysteresis loops of figure (a)...................................................................21

Figure 8. Temperature dependence of magnetization for LSMO/LAO at different

applied magnetic fields. .....................................................................................................22

Figure 9. (a) Surface topography of LSMO (b) MFM phage image of ferromagnetic

LSMO thin film on LAO substrate and (c) surface topography of BZT-BCT film and

(d) PFM phage image of ferroelectric BZT-BCT thin film on LAO substrate ..................24



is a characteristic of the tetragonal crystal structure ..........................................................34

Figure 11. X-ray diffraction pattern for PLD grown LSMO thin film on Pt substrate ......36

Figure 12. Raman spectra of PLD grown thin films (a) BZT-BCT/LSMO on Pt substrate

(black line), BZT-BCT/LSMO on LAO substrate (red line) and (b) .................................37

Figure 13. (a) Magnetization-Applied field (M-H) hysteresis loop of the LSMO/LAO

(red), LSMO/Pt (purple) obtained at room temperature and (b) the low field fragments

of the hysteresis loops of Figure (a) ...................................................................................40

ix

Figure 14. (a) Magnetization-Applied field (M-H) hysteresis loop of the BZT-BCT/

LSMO/LAO (red), BZT-BCT/LSMO/Pt (green) obtained at room temperature and

(b) the low field fragments of the hysteresis loops of Figure (a) .......................................42

Figure 15. (a)FMR absorption derivative vs applied magnetic field for the sample

LSMO/LAO at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow shows the

resonant frequency and the width (orange line) shows the linewidth. (b) Frequency

dependence of FMR linewidth and (c) FMR resonance field for the same sample ...........44

Figure 16. (a) FMR absorption derivative vs applied magnetic field for the sample

LSMO/Pt at 7 GHz (black), 8 GHz (red), and 9.5 GHz (blue). The arrow shows the


dependence of FMR linewidth and (c) FMR resonance field for the same sample ...........46

Figure 17. (a)FMR absorption derivative vs applied magnetic field for the sample BZT-

BCT/LSMO/LAO at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow shows

the resonant frequency and the width (orange line) shows the linewidth. (b) Frequency

dependence of FMR linewidth and (c) FMR resonance field for the same sample. ..........47

Figure 18. (a) FMR absorption derivative vs applied magnetic field for the sample BZT-

BCT/LSMO/Pt at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow shows the


dependence of FMR linewidth and (c) FMR resonance field for the same sample. ..........48

1

OVERVIEW

In the last couple of decades, researchers focused on multiferroic (MF) materials

due to widespread applications of advanced technology1. The MF materials possess two

or more of the ‘ferroic’ order parameters-ferroelasticity, ferroelectricity, and

ferromagnetism2. Some other non-primary order parameters such as ferrotoroidicity3,

antiferromagnetism, ferrimagnetism etc. MF properties exhibit in a material mainly due

to the presence of magneto-electric (ME) coupling. The ME coupling refers to the impact

on the electric polarization through the change in magnetic field or the impact on

magnetization through the change in electric field. The ME coupling is a unique feature

that has a tremendous effect on technology with applications in solid-state transformers,

multiple-state memories4, data-storage media, high sensitivity magnetic field sensors,

actuators, and spintronic5. The goal of the research is to find or develop materials which

would possess both the ferroelectric and ferromagnetic properties in its single phase. But

due to the basic difference in the mechanism of ferroelectricity and ferromagnetism, there

exist very few materials in nature with the MF property4.

The most recognized MF material is BiFeO3 (BFO) which displays the co-

existence of spontaneous electric and magnetic ordering in the same phase at room

temperature6. But for commercial applications with BFO, some limitations must be

overcome. It has a high leakage current and behaves like an antiferromagnetic material

which weakens the magnetization at room temperature. So, the improvement of both the

properties is mandatory for further applications. To enhance the MF properties some

researcher doped rare earth (RE) and transition metal (TM) ions into BFO7. The RE ion

2

as a dopant degrades the ferroelectric polarization8. Therefore, researchers have been

trying to figure out the MF properties of other materials also. Some other reported MF

materials are YMnO39, BiMnO3

10, TbMnO311, DyMnO3

12, and some hexaferrite13. Also,

there are several reports on MF properties of composite materials or heterostructures.

Currently, researchers are working on the heterostructures of BFO/CoFe14 and

BFO/CoFeB15. However, a system has yet to be experimentally demonstrated which will

be electrically and magnetically tunable. In my thesis, heterostructure of ferroelectric

BZT-BCT and ferromagnetic lanthanum strontium manganese oxide (LSMO) has been

used to investigate MF properties.

The precursor of BZT-BCT is BaTiO3 (BTO) which is mainly used as a dielectric

rather than a piezoelectric material because of its poor piezoelectric properties (d33 ∼

200 pC/N) and low Curie temperature Tc ∼ 120oC16. But through doping with suitable

donor or acceptor ions, the Curie temperature and electrical resistance of BTO can be

altered17. Doping of zirconium (Zr) into titanium (Ti) site increases the chemical stability

of the system and doping of calcium (Ca) into barium (Ba) site decreases the

polymorphic phase transition (PPT) temperature significantly in BTO18. Therefore,

BaZrxTi1-xO3 (BZT) and BayCa1-yTiO3 (BCT) have been extensively studied for the future

non-toxic (Pb-free) ferroelectric/piezoelectric materials.

BZT-xBCT is a solid solution of rhombohedral (R3m) BZT and tetragonal

(P4mm) BCT with an MPB (near x = 0.5). Recently, Liu and Ren reported that the bulk

ceramics of a Pb-free BZT-xBCT system has a high piezoelectric coefficient (d33 ∼ 620

pC/N) near the MPB19. Their reported piezoelectricity is comparable to that of Lead

Zirconate Titanate (PZT) and even it is superior to that of existing Pb-free systems. The

3

improved dielectric, piezoelectric and ferroelectric properties are due to the MPB starting

from a tetragonal-cubic-rhombohedral triple point which flattens the energy barrier for

lattice distortion and polarization rotation from (001)T state to (111)R state20. Also, there

might be an intermediate orthorhombic (Amm2) phase, separating the tetragonal and

rhombohedral phases which eases the rotation of ferroelectric domains21.

On the other hand, the ferromagnetic LSMO is a mixed-valence manganite which

is an optimal source of fully spin-polarized carriers and shows a rich physics of magnetic

phases and transport mechanisms22. When lanthanum ions (La3+) in lanthanum

manganese oxide, LaMnO3 are partially substituted with divalent ion Sr2+, a mixed

valence state of Mn3+ and Mn4+ is generated, leading to a dramatic change in physical

properties, such as insulating to metal transition, colossal magnetoresistance (CMR), and

paramagnetic to ferromagnetic transition23. These characteristics can be tuned by

changing Sr dopant concentration in LSMO which prompts applications in magnetic

devices24.

LSMO shows high-spin polarization due to its half-metallic nature arising from

conducting electron. The high-spin feature makes LSMO reliable for applications in spin-

dependent transport devices such as magnetic tunneling junction25, spin-valves26, read

heads in hard disk drive, magnetic random access memory (MRAM), and data transfer in

the magnetic recording system. The switching speed of those applications is limited due

to the magnetic damping of the system. Also, the switching current behavior of the spin-

torque transfer (STT) system27 and the thermal magnetic noise of TMR read heads28

depend on the damping constant. Different applications demand different values of

damping constant. The damping constant for the STT based MRAM must be small to

4

minimize the power consumption. On the other hand, for the read sensors, a large

damping constant is expected to improve the thermal stability29. Therefore, nowadays the

researcher focused to understand the damping mechanism and control parameters to

ensure the applications. In this respect, it is necessary to explore the magnetic dynamics

and to tune the damping properties of LSMO for realizing high-speed spintronic devices.

A considerable number of substrates are commercially available which have

comparable lattice parameters to manganite perovskite materials. They can be used as a

template for the growth of LSMO thin films. Widely used substrates are NdGaO3 (NGO),

(LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT), SrTiO3 (STO), DyScO3 (DSO), and LaAlO3 (LAO).

The lattice parameters are respectively 3.85, 3.87, 3.905, 3.95, and 3.78Å. Among them,

STO, and LSAT are cubic crystals, NGO, and DSO are orthorhombic crystals and LAO

has the rhombohedral distorted perovskite crystal structure, which is a direct match with

LSMO. We have used LAO for the growth of all the thin films as it shows low dielectric

loss and it has minor lattice mismatch with LSMO. Moreover, to investigate the dynamic

interface properties with a conducting material, we deposited LSMO thin film on

platinum (Pt) substrate. The actual structure of the substrate is Pt/Ti/SiO2/Si. The Pt

coated silicon wafers have several advantages. The lattice constant of Pt is 4.0Å, which

means a minimum lattice mismatch with LSMO. In addition, Pt layer is a polycrystalline

layer and hence it helps during crystallization of thin films. It also improves the adhesion

between the substrate and the thin film itself. The additional advantage of Pt is that it has

oxidation resistance capability. The high oxidation temperature of Pt, compared to other

metals as Gold and Silver, make it ideal for high temperature depositions as perovskite

5

phase stabilizes at high temperature only. The intermediate layer Titanium (Ti) is needed

due to less adhesion of Pt to SiO2.

Several factors influence the properties of the thin film such as growth

temperature, background pressure, annealing media, annealing time, oxygen desorption

rate, film thickness, energy density of the excitation source etc. Besides these parameters,

a smooth surface morphology is necessary for the applications of LSMO and BZT-BCT.

Properties of the thin film are very sensitive to the amount of oxygen present during the

deposition. Too much oxygen yield three-dimensional growth while too little oxygen

yields inferior ferromagnetic properties. Structural order plays a vital role to the

stabilization of ferromagnetic and conductive behaviors of the film. Therefore, a tunable

and controlled growth approach is a must. Focusing on ideal layer-by-layer growth can

compromise the functional properties of the film. In this respect, among various

deposition techniques, pulsed laser deposition reveals a well-suited choice.

In this thesis, thin film heterostructures of BZT-BCT(FE)/ LSMO(FM) on two

different substrates-LAO and Pt have been investigated. The heterostructure on LAO

substrate was used to analyze static properties whereas both the heterostructures were

used for dynamic magnetic studies. Structural qualities have been investigated using

XRD and Raman spectroscopy. The ferroelectric layer shows a high degree of

polarization, a low leakage current, and a typical frequency dependent capacitance

behavior. The ferromagnetic layer continues to show its ferromagnetism up to a high

temperature (365K). MFs properties at microscale were investigated using piezoresponse

force microscopy (PFM) and magnetic force microscopy (MFM). FMR spectroscopy has

been used to study the dynamic magnetic properties.

6

FABRICATION AND CHARACTERIZATION OF FERROELECTRIC (FE)-

FERROMAGNETIC (FM) HETEROSTRUCTURES BZT-BCT/LSMO/LAO

USING PULSED LASER DEPOSITION

Abstract

Ferroelectricity and ferromagnetism have been investigated in lead (Pb)-free

0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT)/La0.7Sr0.3MnO3(LSMO)

heterostructures for multiferroic applications. BZT-BCT thin films were grown on

LSMO/ lanthanum aluminate (LAO) by pulsed laser deposition (PLD) technique. Prior to

that, LSMO was deposited on single-crystal LAO substrate by PLD. The epitaxial growth

of the highly-oriented (00l) films was confirmed by XRD pattern. The small FWHM

(0.11o) of the rocking curve peak performed about (00l) peak indicates the better out-of-

plane orientation of the film. The polarization switching behavior has been observed with

a remnant polarization of 93.3 µC/cm2 and a coercive field of 159.89 kV/cm saturates at

5V applied voltage. The low value (0.02) of the dielectric loss confirms the high quality

of the ferroelectric thin film. A well-behaved room temperature M-H curve has been

observed for LSMO/LAO through superconducting quantum interference device

(SQUID) magnetometer indicating the ferromagnetic behavior of the film. The

temperature-dependent magnetization of the film shows paramagnetic to ferromagnetic

transition at about 365K, which is comparable to the bulk samples and recently reported

value as well. These results guide researchers to develop next-generation heterostructures

using BZT-BCT and LSMO for multiferroic applications.

7

Introduction

MF materials simultaneously exhibit ferroelectricity and ferromagnetism in its

single phase1. They can provide the desired ME coupling between the two order

parameters which enable applications in magnetic data storage2, solid-state transformers,

spintronic devices, high sensitivity magnetic field sensors, and actuators3. The researcher

devoted themselves to combine electrical and magnetic properties in one single material

which yielded new MF materials4. Up to today, the most reported MF material is

BiFeO32. Also some other materials such as YMnO3

5. TbMnO36, DyMnO3

7, and

hexaferrites8 attracted much to the researcher. However, researchers reported some

drawbacks such as high leakage current, weak magnetism at room temperature etc. that

limits the MF applications using those materials. To enhance the MF properties some

researcher doped rare earth (RE) and transition metal (TM) ions into BFO9 but the RE ion

as a dopant degrades the ferroelectric polarization10. In addition, there are several reports

on MF properties of composite materials or heterostructures. Currently, researchers are

working on the heterostructures of BFO/CoFe11 and BFO/CoFeB12. However, a system

has yet to be experimentally demonstrated which will be electrically and magnetically

tunable. In this study, I used heterostructure of ferroelectric BZT-BCT and ferromagnetic

LSMO to investigate MF properties. There is no report of heterostructure composed of

that two materials. For MF applications, the study of the properties of such

heterostructure is vital. In this study, the ferroelectric BZT-BCT shows a high degree of

polarization with low leakage current while the ferromagnetic LSMO continues to show

its ferromagnetism up to a high temperature. The shift in the hysteresis loop for the

heterostructure indicated the possible magnetoelectric coupling between the ferroelectric

8

and ferromagnetic layers. Details of the structural property correlation of BZT-

BCT/LSMO/ LAO will subsequently be discussed in the result and discussion section.

Experimental

Ferroelectric BZT-BCT ceramic target was prepared by a standard solid state

reaction using high purity chemicals BaCO3 (99.9%, Inframat Advanced Materials), CaO

(99.95%, Alfa Aesar), TiO2 (99.9%, Sigma-Aldrich), and ZrO2(99.9%, Inframat

Advanced Materials) in appropriate proportions. Polyvinyl alcohol (PVA) was added to

the mixture to prevent agglomeration of the powder particles. The final mixer was

pressed with a ‘hydraulic press’ to form the target. After that, the prepared target was

calcined at 1350 C and then sintered at 1450 C in air. On the other hand, a high purity

dense LSMO target was purchased from Kurt J. Lesker Company. The target was 99.9%

pure, 1.00" diameter×0.250" thick, +/-0.010" AL. A CMP polished highly oriented (001)

LAO substrate (2" dia +/- 0.5 mm × 0.5 thickness +/-0.05 mm) was purchased from MTI

Corporation. The configuration of the substrate was as follows: surface finish (RMS or

Ra): < 8A with free sub-surface damaged, under 1000 class clean room, and in 100 grade

plastic bag in a wafer container.

Thin films of LSMO and then BZT-BCT were deposited on LAO substrates by

Pulsed Laser Deposition (Excel Instrument, PLD-STD-18) technique. A KrF excimer

laser (Lambda Physik, COMPEX 201) with an energy density of 2 Jcm-2, the wavelength

of 248 nm, pulsed duration of 20 ns, was used with 10 Hz pulse rate for the deposition.

Thin films were grown at different growth temperatures (6000C to 8500C) and different

oxygen pressures (10-1 to 10-4 mbar). The best performance was observed in the film

9

grown at the growth temperature of 800oC with 4.1×10-1 mbar oxygen pressure. The base

pressure of the chamber was below 1×10-5 mbar. After the desired number of PLD shot,

the deposited film was cooled down to room temperature maintaining the oxygen

pressure at 110 mbar. The average film thickness was 100 nm, measured ex-situ by a

profilometer (Veeco, Dektak 150).

Thin films were characterized by X-ray Diffractometer (Bruker, D8 Discover)

using θ-2θ scan in the range of 20° to 80° maintaining the Bragg−Brentano reflection

geometry. The excitation source was an x-ray with the wavelength of 1.5405Å. The

vibrational properties of the samples were characterized by micro-Raman scattering

experiments (Horiba Labram Raman-PL System) with a 532 nm green laser excitation

source. The experiments were performed in a back-scattering geometry with 15 seconds

exposure time, and 20 accumulation cycles. The measured spot size was approximately

2.5 μm in diameter on the thin film. We were aware of the possible damage of the thin

film due to the excitation source. The Raman spectroscopy was collected through NGS

Labspec-5 software in the range of 100 cm-1 to 1000 cm-1. The data were analyzed by

Gaussian-Lorentzian peak fitting using Origin Pro 8.5.1.

Temperature and magnetic field dependent magnetization of the films were

measured by a SQUID magnetometer (Quantum Design, MPMS 5XL) interfaced through

MultiVu software. The temperature was varied from 5k to 370k. The M-H hysteresis loop

of the sample was observed by varying magnetic field from -5000 Oe to +5000 Oe. The

maximum sensitivity of the magnetometer was 10-9 emu. Polarization measurement was

carried out by Sawyer-Tower technique. Au-Ti top electrodes were deposited onto the

thin film by thermal evaporation using a physical mask on the thin film. The diameter of

10

the electrodes was 100 to 200𝜇𝑚. The data were analyzed through Origin Pro 8.5.1

software.

Results and Discussion

Figure 1 presents XRD plots of BZT-BCT/LSMO (black) and only LSMO (red)

thin films on LAO substrate. It can be observed that both the samples exhibit a complete

single-phase perovskite structure. There is no evidence of secondary phases such as

Ba3Ca2Ti2O9 and the possibility of such impurity is minimized by sintering at higher

temperatures13. Due to the complete solid solubility of Ca2+ at Ba-site and Zr4+ at Ti-site

at the higher sintering temperature, no other secondary phase was observed in the present

BZT–BCT ceramics.

It should be noted that the highly-oriented thin films demonstrated a weak

splitting, which could be induced by formation of multiple domains, e.g., a- and c-

domains, analogous to the case of PZT14. The splitting of XRD peak in the 2θ range of

43.5 to 44.5 is a unique characteristic of the tetragonal phase in BZT-BCT. The single

(002) plane corresponding to cubic phase transforms into (200) and (002)

crystallographic planes, which are the characteristics of a tetragonal structure15. The inset

XRD plot in Figure 1 corroborates the presence of tetragonal crystallographic structural

symmetry in the PLD grown thin film. The asymmetricity of the peak shape indicates the

existence of tetragonal and cubic phases. This tetragonal structure might partly be a

consequence of averaging ⟨111⟩ displacements of octahedral Ti4+ (local rhombohedral

structure)16. The splitting of peaks must not be attributed to the presence of CuKα or

CuKβ at higher

11

angles, as this is predominantly seen in the mentioned 2θ range (43.5 to 44.5). In

addition, the (003) diffraction peak at 68.5º shows mixed diffraction peaks, which is

consistent with Liu’s results17. Such coexistence plays a key role in enhancing the

piezoelectric performance. To further investigate the texture of the films, we performed

XRD rocking curve around BZT-BCT (002), BZT-BCT (003), and LSMO (002) plane,

which are displayed in Figure 2(a), 2(b), and 2(c) respectively. The FWHM (θ) of the

rocking curves are 0.10°, 0.14°, and 0.37° respectively. The small values of FWHM

confirmed the oriented nature of the films.

The c-axis lattice constants of LSMO calculated from the XRD data for LSMO/

LO and BZT-BCT/LSMO/LAO are 3.892 and 3.943 Å, respectively. As the lattice

20 30 40 50 60 70 80

*

*

*

Inte

nsi

ty (

a.u

.)

2(degree)

BZT-BCT/LSMO/LAO

LSMO/LAO

*

LAO

LSMO

BZT-BCT

(001) (002)

(003)

43.5 44.0 44.5

Experimental data

(200) peak

(002) peak

Fitted data

F

D



is a characteristic of the tetragonal crystal structure.

12

constant of bulk LSMO is 3.889 Å, this induces tensile strain in the LSMO thin film.

However, the scenario is critical for the heterostructure. The BZT-BCT/LSMO/ LAO

heterostructure experiences two strain effects, one incurred from the strain of LSMO/

LAO due to the lattice mismatch between the film and the substrate and the other from

the BZT-BCT/LSMO interface. Table 1 shows the experimental and standard values of

the c-axis lattice parameters.

In addition to XRD analysis, we performed room-temperature Raman experiment

to investigate the molecular vibrational modes present in the crystal lattice. Raman

spectroscopy is a highly responsive technique to evaluate the atomic structure and phases

of the materials. The cubic perovskite structure inherently has no Raman active modes

while the tetragonal structure shows some significant modes18. The phases of ABO3 type

44.1 44.2 44.3 44.4 44.5 44.6

FWHM () = 0.10o

Inte

nsi

ty (

a.u

.)

2(degree)

BZT-BCT(002)(a)

68.4 68.6 68.8 69.0 69.2

Inte

nsi

ty (

a.u

.)

2 (degree)

BZT-BCT(003)

FWHM () = 0.145o

(b)

45.5 46.0 46.5 47.0

Inte

nsi

ty (

a.u

.)

2(degree)

LSMO(002)

FWHM()=0.37o

(c)

Figure 2. High resolution rocking curve profiles for the (a) BZT-BCT (002) (b) BZT-

BCT (003), and (c) LSMO (002) reflections. Horizontal axes are shown in the relative

angles of 2θ in degree. Vertical axes are shown in the arbitrary units. The FWHM (θ)

of the reflections are 0.10o, 0.145o, and 0.37o, respectively.

Table 1. Experimental out-of-plane lattice parameters from XRD (all the values in

Angstrom)

BZT-BCT LSMO LAO

LSMO/LAO

3.892 3.802

BZT-BCT/LSMO /LAO 4.117 3.943 3.802

13

perovskite crystals are interesting. For example, tetragonal BaTiO3 has five atoms and

there are fifteen degrees of freedom present per unit cell19. It behaves as a ferroelectric

crystal below its transition temperature (Tt ~120oC) and the vibrational modes are Raman

active up to Tt. The cubic phase above Tt is essentially Raman inactive and can be easily

distinguishable from the Raman active tetragonal structure20. The Raman spectrum of

tetragonal BZT-BCT thin film shown in Figure 3 exhibits the following six active modes:

2 A1(TO), 1 A1(LO), 1 E(TO), 1 E(LO), and 1 B1. Each of the A1 and E modes split into

TO (transverse optical) and LO (longitudinal optical) modes due to the presence of long-

range electrostatic forces21.

The signature Raman peaks in the BZT-BCT thin film are observed at 179.94,

320.51, 543.35, and 748.31 cm-1. The peak at 179.94 cm-1 corresponds to the A1 (TO)

mode of vibration22 which represents the Ti-O phonon vibrations of BZT-BCT. Another

A1 (TO) mode at around 543.35 cm-1 represents the O-Ti-O symmetric stretching

200 400 600 800 1000

Inte

nsi

ty (

a.u

.)

Raman Shift(cm-1)

BZT-BCT/LSMO/LAO

Fitted Curve

A1(LO)/E(LO)

A1(TO)

E(TO)/B1

A1(TO)

Figure 3. Raman spectra of BZT-BCT/LSMO thin film on LAO substrate with

Gaussian and Lorentzian peak fitting by using Origin software.

14

vibrations23. These vibrations are caused due to the presence of non-centrosymmetric

regions in which a Ti or Zr atom is displaced from its octahedral position because first-

order Pm3m Raman scattering is symmetry-forbidden in the centrosymmetric space

group24. The sharp feature at 543.35 cm-1 indicates the high quality of the thin film. The

peak at 320.51 cm-1, attributed to the B1 mode, is a characteristic peak for BZT–BCT and

indicates the asymmetry of the TiO6 octahedra25. This band is considered to be the

Raman signature of the tetragonal phase. The interesting feature in the Raman spectra of

our BZT-BCT thin film is the absence of the A1g octahedral breathing mode at 800 cm-1

which was observed in their bulk counterparts26. The breathing mode is associated with

more dissimilar ions on B-site of the Raman active perovskite27. The absence of breathing

mode demonstrates that Ca2+ resides on the Ba-site, not on the Ti-site in our case.

The broadening and peak shift of A1 (LO) vibrational mode at 748.31 cm-1,

compared to its bulk counterpart, is a characteristic feature of the tetragonal phase. The

broadening represents the distortion in the thin film. Both the peak position and the

broadening of this mode are related to the non-centrosymmetric region in the unit cell.

The weak intensity of some of the vibration bands of the BZT–BCT film denotes the

heavily damped phonons. The A1 (TO) mode at 543.35 cm-1 and A1 (LO) mode at 748.31

cm-1 have a vital contribution in the polarization kinetics. The displacement of Ti/Zr ions

from its octahedral position causes an increase in the dipole moment of the unit cell

which finally increases the polarization.The peak positions of some of the modes such as

B1/E (TO) and A1 (LO) / E (LO), shift towards either the lower or higher frequency

region. This phenomenon of upshift/downshift in Raman spectrum indicates the localized

chemical environments and/or strain19. As the BZT-BCT thin films were deposited under

15

the same PLD conditions, the deviation of chemical composition can be ruled out which

has also been confirmed by XRD analysis. So, the epitaxial strain originating from both

the substrate and the LSMO layer responsible for the shifting of Raman modes. Yamada

et al. explained the strain relaxation process by surface energy variation in the different

crystallographic surface in Barium Strontium Titanate (BST) system28. The LSMO thin

film in our study yields a much lower vibrational frequency to be detected in the Raman

spectroscopy due to its half-metallic nature.

The well-defined peaks in the Raman spectroscopy prove the high quality of the

thin film. Taken together, the XRD and Raman spectroscopy results indicate the presence

of non-centrosymmetric regions that result from the local off-centering of the titanium

(zirconium) atoms. Table 2 represents the peak positions and full width at half maximum

(FWHM) of the corresponding vibrational modes in the BZT-BCT thin film.

The polarization (P) vs electric field (E) hysteresis loop is the proof of the

ferroelectricity in a material. Figure 4 shows the experimental P-E curves of an Au-

Ti/BZT-BCT/LSMO ferroelectric capacitor. The curves are nothing but a hysteresis loop

which is the indication of the ferroelectricity in the thin film. The hysteresis loops are

almost centered along the y-axis. As the voltage increases, the capacitor starts to show

hysteric characteristics and saturates at a higher voltage. Table 3 shows the remnant

polarization, saturation polarization along with the applied voltage and coercive field for

the sample Au-Ti/ BZT-BCT/LSMO ferroelectric capacitor. The experimental values are

mentioned with their units.

16

The P-E hysteresis loops yield a large remnant and saturation polarization in our

BZT-BCT thin film. It showed a maximum remnant polarization (Pr) of 93.3 µC/cm2 and

a coercive field (Ec) of 159.89 kV/cm for the MPB composition at 5V applied voltage.

The reason behind the high remnant polarization in the PLD grown thin film is the


in Raman spectroscopy

Peak Position

(cm-1)

Vibration mode FWHM

(cm-1)

Reason

179.94 A1(TO) 518.25 Ti-O phonon vibration

320.51 E(TO)/B1 108.61 Raman signature

543.35 A1(TO) 162.36 O-Ti-O symmetric stretching

vibrations;

748.31 A1(LO)/E(LO) 198.21 Raman signature

-600 -400 -200 0 200 400 600-150

-100

-50

0

50

100

150

1 Volt

2 Volt

3 Volt

4 Volt

5 Volt

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)

Figure 4. Polarization-electric field (P-E) hysteresis loops for ferroelectric capacitors

Au-Ti/BZT-BCT/LSMO at different applied voltages

17

uniform distribution of grain sizes, compositional homogeneity, absence of domain-wall

pinning centers, and low defect density as described previously29.

The observed coercive field was also higher than that of the BZT-BCT bulk

ceramic (1.68 kV/cm)30. The much higher coercive field is a consequence of much

smaller grains and substrate clamping effect31. As the average grain size is small in PLD

grown thin films compared to their bulk counterpart, the length of the grain boundaries

per unit volume is higher. This requires higher electric field to orient all the domains

along the field direction. The orientation-dependent analysis by Luo et al. demonstrated

that (001) oriented thin film shows superior ferroelectricity rather than (110) and (111)-

oriented thin film30. In this work, our sample is highly oriented along (001) direction,

which upholds the high polarization phenomena in the BZT-BCT samples.

The dielectric properties of the BZT–BCT thin film were measured at room

temperature as a function of frequency ranging from 1 kHz to 50 kHz with an exciting

voltage of 1 V. Both the electrodes were thermally evaporated Au-Ti. The top electrode

Table 3. Remnant and saturation polarization with coercive field at different voltages

applied to FECAP

Applied Voltage

(volt)

Remnant

Polarization,

Pr (µC/cm2)

Saturation

Polarization,

Ps (µC/cm2)

Coercive Field,

Ec (kV/cm)

1 4.36 26.2 5.87

2 14.3 55.1 23.31

3 28.8 85.8 46.13

4 56.3 122 99.14

5 93.3 163 159.89

18

was on the ferroelectric layer whereas the bottom electrode was taken from the half

metallic LSMO layer. Figure 5(a) and 5(b) represent the typical frequency dependent

dielectric constant and dielectric loss in a wide range of frequencies respectively. With

increasing frequency, dielectric constant decreases but the loss tangent increases. The

measured largest capacitance is 2.79 nF which yields a high dielectric constant of 5100

because the spontaneous polarization vector points are normal to the (001)-oriented film

surface and coincide with the dielectric measuring direction32.

The leakage current and the time-dependent dielectric break-down (TDDB) are

vital issues for applications in electronics and memory devices. For a capacitor, which

should completely block direct current (dc), the leakage current is the amount of dc

current that flows at some dc bias. In a different way, we can say that the leakage

phenomenon is a gradual loss of energy from a charged capacitor. The electronic devices

have different regions of operation, such as active region, saturation region, cut-off

region etc. The leakage current is the small amount of current that flows through the

device even the

0 10 20 30 40 50

4850

4900

4950

5000

5050

5100

5150

di-

ele

ctr

ic c

on

stan

t

Frequency (kHz)

BZT-BCT/LSMO/LAO

Top electrode: Au-Ti

Bottom electrode: LSMO

T = 300K (a)

3.0 3.5 4.0 4.50.00

0.02

0.04

0.06

0.08

0.10

Sample: BZT-BCT/LSMO/LAO

Loss

tan

gen

t

Log10 f

(b)

Figure 5. Frequency dependence of dielectric constant (a) and dielectric loss (b) for

BZT–BCT/LSMO thin film on LAO substrate. A wide range of frequencies was

applied from 1KHz to 50KHz. The dielectric loss was plotted against logarithm of the

frequency.

19

device is ‘turned off’. Though, this current is really small compared to ‘on current’, it still

slowly discharges the capacitor. In leakage analysis, the time to breakdown (tBD) is

another important characteristic of the device. This time is generally measured by

applying a variable voltage until dielectric breakdown. It is usually assumed that tBD is a

function of the total number of charged carriers passing through the films. Therefore, tBD

is inversely proportional to the leakage current density (J)33.

Most of the literature have demonstrated the ferroelectric properties and leakage

behavior of BZT-BCT deposited on the metal electrode (for example Pt)34. Hwang et al.

reported that, in general, thin films on metal electrodes exhibit lower leakage current

density and withstand higher dc voltage without a dielectric breakdown in comparison to

oxide electrodes35. Our ferroelectric material was deposited on oxide material (LSMO)

and we have used that as the bottom electrode. So, it is a must to determine the leakage

behavior of BZT-BCT with LSMO as an electrode.

Figure 6 depicts the typical room temperature leakage current density-time (J-t)

profiles assessed at two different applied electric fields of 100 kV/cm and 400 kV/cm for

the BZT-BCT thin film. Both the current densities show similar behavior with respect to

time passes. Initially, the leakage currents increase gradually with time and after a while

reached a saturated steady state. The leakage current at low electric field reaches steady

state faster than that at the higher electric field. The steady state values of the leakage

currents at low and high fields are 15.9 and 78.1 µA/cm2 respectively which are very

much comparable with reports on ferroelectric material with a metal electrode36. The

ferroelectric material is ideally expected to possess very high electrical resistance.

However, several factors such as cationic/anionic vacancies, structural distortions, grain

20

boundaries, film-electrode interface, etc., act as low resistance channels, thereby

contributing to the mobility of charges in terms of leakage current. In our sample, as the

leakage currents are very small, a higher time to breakdown is expected of our

heterostructure.

PLD grown LSMO thin film was highly epitaxial stimulated by monocrystalline

LAO substrate at higher deposition temperature. Figure 7(a) shows the M-H curve of

LSMO/LAO thin films with the field applied parallel to the film plane at room

temperature. Figure 7(b) shows the low field fragments of the hysteresis loops. The

magnetization was calculated after subtraction of a diamagnetic background from the

substrate. The magnetization increases with increasing magnetic field until saturation is

0 200 400 600 800 1000

30 60 90 120 150

Cu

rren

t d

ensi

ty (

a.u

.)

Time (ms)

Electric Field: 400 kV/cm

Cu

rren

t d

ensi

ty (

A/c

m2)

Time (ms)

100 kV/cm

400 kV/cm

80

70

10

Figure 6. Leakage current characteristics of Au-Ti/BZT-BCT/LSMO with two

different applied electric fields 100 kV/cm (black line) and 400 kV/cm (red line).

21

reached at 500 Oe. A well-behaved M-H curve was observed for the LSMO thin film

which is the indication of ferromagnetism in the sample. It is worth noting that the

magnetic disorder at the surface has been suggested to be considerably larger than in the

bulk. In perovskite material, the cubic symmetry lacks at surfaces and charge is

transferred from the bulk to the surface layers, leading to the formation of Mn3+37.

Figure 8 shows the temperature dependence of magnetization (M-T curve) of the

LSMO/LAO sample at two different magnetic fields (High field of 5000 Oe and Low

field of 100 Oe). The measurements were performed in a field-cooled condition, known

as FC magnetization. Both the M-T curves reveal the Curie temperature (Tc) of the

sample. The commencement of a spontaneous magnetization near T = 365K indicates Tc

of the ferromagnetic thin film. Table 4 summarizes values from SQUID measurement.

The surface morphology (topography) of the LSMO thin film deposited on LAO

substrate is shown in Figure 9(a). There are no distinctive domain structures and domain

walls found in the topography. However, the color contrast reveals the existence of

-400.0 -200.0 0.0 200.0 400.0

Mag

net

iza

tion

(n

orm

ali

zed

)

Magnetic Field, H (Oe)

LSMO/LAO

BZT-BCT/LSMO/LAO

T = 300K

(a)

-20 -10 0 10 20 30

-0.4

-0.2

0.0

0.2

Ma

gn

etiz

ati

on

(n

orm

ali

zed

)

Magnetic Field, H (Oe)

LSMO/LAO

BZT-BCT/LSMO/LAO

T = 300K

(b)

Figure 7. (a) Magnetization-Applied field (M-H) hysteresis loop of LSMO/LAO (red),

BZT-BCT/LSMO/LAO (purple) obtained at room temperature and (b) the low field

fragments of the hysteresis loops of figure (a)

22

different domains in the sample. The MFM phase image is shown in Figure 9(b). The

image contrast of the phase image is a clear indication of the magnetic activity present in

the sample. The contrast shown by the MFM image occurs because of force gradients

between the FM tip and the magnetic activity present on the sample’s surface. The phase

image was achieved after topography measurements (tapping mode) followed by sample

surface scanning at a constant height (lift mode). According to this procedure, no van der

0 100 200 300 400

0.0

20.0k

40.0k

60.0k

80.0k

FC

Ma

gn

etiz

ati

on

(em

u/c

m3)

Temperature (K)

H = 5000 Oe

H = 100 Oe

TC ~ 360K(FC Curve)

Figure 8. Temperature dependence of magnetization for LSMO/LAO at different

applied magnetic fields.

Table 4. Results from SQUID measurement

Results from M-H hysteresis loop Results from M-T curve

Saturation Magnetization, 32 kemu/cm3 Field Curie Temperature

Remnant Magnetization, Mr 11 kemu/cm3 H = 5000 Oe TC = 365.2 k

Coercive field, Hc 15.19 Oe H = 100 Oe TC = 354.18 k

23

Waals forces are expected to be detected, and any change in the vibration amplitude of

the cantilever is proportional to the gradient of magnetic fields perpendicular to the

sample surface.

The surface morphology of BZT-BCT film is shown in Figure 9(c). There is no

evidence of cracking or defects on the surface. Furthermore, the topography is very much

uniform which is an indication of a homogenous PLD deposition. From PFM surface

morphology observation, it can be inferred that the BZT–BCT film exhibits a uniform

and dense microstructure. Moreover, based on the substrate orientation dependence of the

equilibrium grain shapes of the BZT-BCT thin film and the Winterbottom construction

theory, it could be visualized that our BZT-BCT thin film maintains a layer-by-layer

growth along the lowest energy surface for (001)-orientation38. The PFM phase image for

the BZT-BCT thin film was obtained utilizing commercially available conducting tip in

contact mode by applying a DC voltage of 1 volt between the tip (top electrode) and the

bottom electrode (LSMO) grounded. The out-of-plane PFM phase image is shown in

Figure 9(d). The phase image distinguishes the domains of opposite polarization with

dark and bright contrast in the image. After scanning with a signal to the tip with respect

to sample surface, a piezoelectric contrast (shown by the bright and dark shades) appears,

result in spontaneous polarization, which can be switched by applying an external

excitation. Therefore, we can conclude that the ferroelectricity in the BZT–BCT film at

the nanoscale level can be confirmed using PFM.

24

Conclusions

Highly oriented ferroelectric-ferromagnetic heterostructure was fabricated using

PLD. Along with the theta-2theta scan of XRD, the small full width at half maximum

(FWHM) at high-resolution XRD rocking curve corroborates the oriented nature of the

films. The BZT-BCT ferroelectric thin film shows polarization switching behavior with a

large remnant polarization of 93.3 μC/cm2, a high coercive field of 159.89 kV/cm, and a

large dielectric constant with negligible loss. The well-behaved M-H hysteresis loop at

room temperature confirms the ferromagnetic nature of the LSMO thin film with high

Figure 9. (a) Surface topography of LSMO (b) MFM phage image of ferromagnetic

LSMO thin film on LAO substrate and (c) surface topography of BZT-BCT film and

(d) PFM phage image of ferroelectric BZT-BCT thin film on LAO substrate.

25

saturation and remnant magnetization. Temperature dependent magnetization data

confirm a high Curie temperature of 360k, which is very close to the bulk sample.

Further, PFM and MFM of the ferroelectric and ferromagnetic thin film, respectively

reveal the corresponding domain of the as-grown film. These results may guide

researchers to develop next-generation Pb-free ferroelectric-ferromagnetic

heterostructures using BZT-BCT and LSMO for applications with electro-mechanical,

and magneto-electric coupling.

Acknowledgement

I would like to acknowledge Dr. Gail J Brown, Air Force Research Laboratory

(AFRL), Wright-Patterson AFB, OH 45433, USA for providing support to collect the

polarization data.

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Adv. Mater. 18, 2747–2752 (2006).

29

FERROMAGNETIC RESONANCE STUDY OF BZT-BCT/ LSMO

HETEROSTRUCTURE GROWN ON DIFFERENT SUBSTRATES-LAO AND

PLATINUM USING PULSED LASER DEPOSITION

Abstract

In this article, dynamic magnetic properties of La0.7Sr0.3MnO3 (LSMO) capped

with a Pb-free ferroelectric BZT-BCT layer deposited on two different substrates

lanthanum aluminate (LAO) and Platinum (Pt) by pulsed laser deposition (PLD) have

been investigated by FMR spectroscopy. The thin films on LAO substrate were highly

(00l)-oriented whereas for Pt substrate, the films were randomly oriented. XRD data

confirm the epitaxial growth of the thin films. The vibrational modes of the ferroelectric

layer, investigated through Raman spectra, reveal significant red shifts for the thin films

on Pt substrate compared to films on LAO substrate. The well-behaved room temperature

M-H curves were observed for all the films through superconducting quantum

interference device (SQUID) magnetometer indicating the ferromagnetic behavior of

LSMO. The right shift of the hysteresis loop of the heterostructure may arise from the

ME coupling between the ferroelectric and ferromagnetic layers. FMR measurement

yields the significant values of linewidth offset, Gilbert damping parameter,

gyromagnetic ratio, and in-plane uniaxial anisotropy field of the thin films. We found the

lowest Gilbert damping parameter 0.02883 for the heterostructure BZT-BCT/LSMO/

LAO. In addition, the gyromagnetic ratio was also found lowest (0.00169 GHz/Oe) for

the same film. The analysis concludes possible spintronic applications using BZT-

BCT/LSMO on LAO substrate rather than films on Pt substrate.

30

Introduction

The precursor material of LSMO is lanthanum manganite (LaMnO3). After

doping Sr2+ into LaMnO3 through solid-state reaction, LSMO is formed and it shows

some amazing properties far away from LaMnO3. The unique properties are promising

for potential applications in magnetic, magnetoelectronic, and photonic devices as well as

in infrared detector and spintronic technology. The interplay among the spin, charge, and

orbital degrees of freedom in this material1 leads to many intriguing phenomena such as

metal-insulator transition, magnetic phase transition, and nanoscale electronic phase

separation2.

LSMO belongs to the family of perovskite like mixed valence manganites which

is the most studied colossal magnetoresistant (CMR) material. The half-metallic nature of

LSMO yields a high-spin polarization of its conducting electrons. This property makes it

a reliable material for spin-dependent transport devices such as spin valves3, magnetic

tunneling junctions4 especially when it is grown on strontium titanate (SrTiO3 or STO)

and neodymium gallate (NdGaO3 or NGO) substrates. LSMO has nearly 100% spin

polarization and a high Curie temperature of 365k. The other applications of LSMO

involve magnetic random access memory (MRAM), read heads in hard disk drives, as

well as spin-logic based devices5. Moreover, there are more advanced applications of

magnetic materials in data transfer in magnetic recording systems. The switching speed

of the magnetic elements for those applications is limited, in part, by the magnetic

damping in the thin film. An understanding of the damping mechanisms and control

parameters remains one of the key challenges in the push to achieve faster switching

speeds. In this respect, it is a must to understand the magnetic dynamics and to tune the

31

damping properties of LSMO for realizing high-speed spintronic devices. The damping

constant determines the critical switching current in a spin-torque transfer (STT) based

devices6. For STT based MRAM, damping constant must be small to minimize the power

consumption. Also, for TMR read heads, the thermal magnetic noise depends on the

damping parameters which affects SNR of a read head7. On the other hand, a large

damping constant is expected for the read sensors to improve the thermal stability8.

All the above mentioned applications required LSMO interfaced with either an

insulator or conducting or semiconducting materials. But the properties of LMSO is

changed due to the interface and it makes the device applications limited9. In that respect,

understanding the surface and interface properties of LSMO for magnetic and transport

mechanism is vital. The properties of LSMO interfaced with a ferroelectric material is

promising for various applications. Specially, how the magnetization of an LSMO thin

film is altered when epitaxially joined to a ferroelectric material is very important.

Interface surface pinning, unidirectional and uniaxial anisotropies and changes to other

micromagnetic properties are all key characteristics for tunnel junction performance.

Several reports demonstrated these properties in single-layer LSMO10. The effects of

ferroelectric overlayer can be important and have begun to be studied11. In this report, we

have shown the FMR study of LSMO thin film deposited on different substrates-LAO

and Pt. Also, we have demonstrated the change of FMR properties after interfacing with a

ferroelectric material BZT-BCT. The low values of Gilbert damping parameter with the

typical values of gyromagnetic ratio and in-plane uniaxial anisotropy field confirm the

possible applications in high-speed spintronic devices.

32

Experimental

All chemicals were analytic grade reagents and used without further purification.

The BZT-BCT ceramic target was prepared by a standard solid state reaction using high

purity chemicals BaCO3 (99.9%, Inframat Advanced Materials), CaO (99.95%, Alfa

Aesar), TiO2 (99.9%, Sigma-Aldrich), and ZrO2 (99.9%, Inframat Advanced Materials) in

appropriate proportions. Polyvinyl Alcohol (PVA) was added to the mixture to prevent

agglomeration of the powder particles. The final mixer was pressed with a ‘hydraulic

press’ to form the target. After that, the prepared target was calcined at 1350oC and then

sintered at 1450oC in the air. The CFO target was prepared from CoFe2O4 nanopowder

(Inframat Advanced Materials) applying the same procedure with 900oC sintering

temperature. On the other hand, a high purity dense Lanthanum Strontium Manganite

(La0.7Sr0.3MnO3) (LSMO) target was purchased from Kurt J. Lesker Company. The target

was 99.9% pure, 1.00" diameter×0.250" thick, +/-0.010" AL.

A CMP polished highly oriented (001) LAO substrate (2" dia +/- 0.5 mm × 0.5

thickness +/-0.05 mm) was purchased from MTI Corporation. The configuration of the

substrate was as follows: surface finish (RMS or Ra): < 8A with free sub-surface

damaged, under 1000 class clean room, and in 100 grade plastic bag in a wafer container.

The platinum substrate was also bought from MTI corporation with following

specifications: Film-SiO2+Ti+Pt (111) thin film on Si (100) (P-type) substrate,

2"x0.279mm, 1 side polished (1sp), SiO2=300 nm, Ti=10 nm, Surface roughness: < 20 A

RMS.

All the thin films were deposited on LAO and Pt substrate by Pulsed Laser

Deposition (Excel Instrument, PLD-STD-18) technique using those targets. A KrF

33

excimer laser (Lambda Physik, COMPEX 201) with an energy density of 2 Jcm-2,

wavelength 248 nm, pulsed duration of 20 ns, was used with 10 Hz pulse rate for the

deposition. Thin films were grown in different growth temperature (6000C to 8500C) and

different oxygen pressure (10-1 to 10-4 mbar). The best performance was observed in the

film grown at the growth temperature of 800oC with 4.1×10-1 mbar oxygen pressure. The

base pressure of the chamber was maintained at 9×105 mbar. After the desired number of

PLD shots, the deposited film was cooled down to room temperature maintaining the

oxygen pressure at 110 mbar. The average film thickness was 100 nm, measured ex-situ

by a profilometer (Veeco, Dektak 150).

Thin films were characterized by X-ray Diffractometer (Bruker, D8 Discover)

using θ-2θ scan in the range of 20° to 80° maintaining the Bragg−Brentano reflection

geometry. The excitation source was an x-ray with the wavelength of 1.5405Å. The

vibrational properties of the samples were characterized by micro-Raman scattering

experiments (Horiba Labram Raman-PL System) with a 532 nm green laser excitation

source. The experiments were performed in a back-scattering geometry with 15 seconds

exposure time, and 20 accumulation cycles. The measured spot size was approximately

2.5 μm in diameter on the thin film. We were aware of the possible damage of the thin

film due to the excitation source. The Raman spectroscopy was collected through NGS

Labspec-5 software in the range of 100 cm-1 to 1000 cm-1. The data were analyzed by

Gaussian-Lorentzian peak fitting using Origin Pro 8.5.1.

The magnetization of the films was measured by SQUID mangnetometer

(Quantum Design MPMS 5XL) interfaced through MultiVu software. The temperature

was varied from 5k to 370k. Low temperature (5k) and high temperature (300k) M-H

34

hysteresis loop of the sample was observed by varying the magnetic field from -5000 Oe

to +5000 Oe. The maximum sensitivity of the magnetometer was 10-9 emu. In our

experiment, we used a slow and steady temperature ramp and sent a constant current

using the current source.

Results and Discussion

Figure 10 illustrates the XRD pattern of both the BZT-BCT/LSMO (black) and

only LSMO (red) thin films on LAO substrate. The XRD pattern shows the highly-

oriented spectra which are the indication of a complete single phase perovskite structure.

Puli et al. demonstrated a possible secondary phase such as Ba3Ca2Ti2O9 in the (1-x)

BZT-xBCT ceramic system12. But in our sample the existence of any secondary phases is

ruled out due to sintering at a higher temperature. Also, the complete solid solubility of

20 30 40 50 60 70 802

3

4

5

6

7

*

*

*

Inte

nsi

ty (

a.u

.)

2(degree)

BZT-BCT/LSMO/LAO

LSMO/LAO

(Log Plot)

*

LAO

LSMO

BZT-BCT

(001) (002)

(003)

43.5 44.0 44.5

Experimental data

(200) peak

(002) peak

Fitted data

F

D



is a characteristic of the tetragonal crystal structure.

35

Ca2+ at Ba-site and Zr4+ at Ti-site at higher sintering temperature ruled out the possibility

of any impurities, confirmed by XRD.

The interesting feature of the XRD pattern is the splitting of the peaks. The inset

of Figure 10 shows that weak splitting of the XRD peak in the 2θ range of 43.5 to 44.5

induced by the formation of multiple domains, e.g., a- and c-domains, analogous to the

case of PZT13. This is a unique characteristic of the tetragonal phase in BZT-BCT as the

single (002) plane (cubic phase) transforms to (200) and (002) crystallographic planes

(tetragonal phase). The other interesting feature of the XRD is the asymmetricity of the

peak shape which confirms the coexistence of tetragonal and cubic phases in our thin

film. The average ⟨111⟩ displacements of octahedral Ti4+ (local rhombohedral structure)

is the reason of the tetragonal structure14. The presence of CuKα or CuKβ at higher angles

can’t be the source of the peak splitting as this is predominantly seen in the mentioned 2θ

range. The other diffraction peak (003) at 68.5o also shows mixed diffraction peaks,

which is consistent with Liu’s report15. Such coexistence plays a key role in enhancing

the ferroelectric performance of BZT-BCT.

The c-axis lattice constants, of LSMO, calculated from the XRD data for

LSMO/LAO and BZT-BCT/LSMO/LAO are 3.892 and 3.943 Å respectively. As the

lattice constant of bulk LSMO is 3.889 Å, this induces tensile strain in the LSMO thin

film. However, the scenario is critical for the heterostructure. The BZT-BCT/LSMO/

LAO heterostructure experiences two strain effects, one incurred from the strain of

LSMO/LAO due to the lattice mismatch between the film and the substrate and the other

from the BZT-BCT/LSMO interface.

36

Figure 11 represents the XRD pattern of the LSMO thin film grown on Pt

substrate, which reveals the randomly oriented nature of the film. The XRD peaks at

23.04, 32.5, 47.5, and 69.83 correspond to LSMO (001), LSMO (011), LSMO (002), and

LSMO (003) respectively. The other peak at 40.03 denotes the reflection from Pt

substrate.

To investigate the molecular vibrational modes present in the sample, we

performed room-temperature Raman experiment. This spectroscopy is a highly sensitive

technique to evaluate the atomic structure and phases of perovskite BZT-BCT. ABO3

type perovskite structures are very interesting to investigate. For example, Raman active

modes are only found in the tetragonal perovskite structure rather than in cubic

structure16. BaTiO3 (BTO), a perovskite, is the primary material of the BZT-BCT

composite. There are fifteen degrees of freedom present per unit cell (uc) of tetragonal

BTO as it has five atoms in a uc17. The transition temperature (Tt) of BTO is 120oC. The

20 30 40 50 60 70 80

LS

MO

(002)

LS

MO

(003)

LS

MO

(01

1)

Pt(111)

LS

MO

(001)

Inte

nsi

ty (

a.u

.)

2(degree)

LSMO/Pt

Figure 11. X-ray diffraction pattern for PLD grown LSMO thin film on Pt substrate

37

tetragonal crystal structure of BTO transforms to cubic crystal structure upon heating

above Tt. In addition, the ferroelectric property vanishes at this temperature. The cubic

BTO is not Raman active while the tetragonal structure shows significant modes18. Figure

12(a) shows the following six active modes in Raman spectra of the BZT-BCT/LSMO

thin film on both the LSMO and Pt substrate: 2 A1(TO), 1 A1(LO), 1 E(TO), 1 E(LO),

and 1 B1. Due to the presence of long-range electrostatic forces19 each of the A1 and E

modes split into TO (transverse optical) and LO (longitudinal optical) modes. Figure

12(b) represents the deconvolution of the vibrational modes of BZT-BCT.

The peaks in the BZT-BCT thin film on both the substrates are observed near 175,

300, 525, and 735 cm-1. The peak near 175 cm-1 represents the Ti-O phonon vibrations of

BZT-BCT and denoted to the A1(TO) mode20. Another A1(TO) mode at near 525 cm-1

represents the O-Ti-O symmetric stretching vibrations21. The first-order Pm3m Raman

scattering is symmetry-forbidden in the centrosymmetric space group. So, due to a Ti or

200 400 600 800

A1(LO)/E(LO)

A1(TO)

E(TO)/B1

A1(TO)

In

ten

sity

(a.u

.)

Raman Shift(cm-1)

BZT-BCT/LSMO/Pt

BZT-BCT/LSMO/LAO

(a)

200 400 600 800 1000

Inte

nsi

ty (

a.u

.)

Raman Shift(cm-1)

BZT-BCT/LSMO/LAO

Fitted Curve

A1(LO)/E(LO)

A1(TO)

E(TO)/B1

A1(TO)(b)

Figure 12. Raman spectra of PLD grown thin films (a) BZT-BCT/LSMO on Pt

substrate (black line), BZT-BCT/LSMO on LAO substrate (red line) and (b)

Deconvolution of Raman spectra with Gaussian and Lorentzian peak fitting by using

Origin Pro 8.5.1

38

Zr atom’s displacement from its octahedral position, the presence of non-

centrosymmetric regions yield the symmetric stretching vibrations22. The sharp feature of

the peak near 525 cm-1 indicates the high quality of the thin film. The characteristic peak

for BZT–BCT, near 300 cm-1, is attributed to the B1 mode. This mode indicates the

asymmetry of the TiO6 octahedra23. This band is also considered to be the Raman

signature of the tetragonal phase. The A1g octahedral breathing mode at 800 cm-1 which is

absent in the Raman spectra which confirm that Ca2+ resides on Ba-site not Ti-site in our

cases24.

A1 (LO) vibrational mode near 735 cm-1 is quite broad and shifted compared to its

bulk counterpart. This is a characteristic feature of the tetragonal phase in thin films. The

broadening represents the distortion in the thin film while the peak shift is related to the

non-centrosymmetric region in the unit cell. Some of the Raman modes in our sample are

weak due to heavily damped phonons.

The BZT-BCT thin films in our study were epitaxially grown on conducting

oxide (LSMO), which was earlier grown on different substrates (Pt and LAO). The LAO

substrate is insulating while the Pt one is conducting. So, in the sample BZT-BCT/

LSMO/Pt, obviously, the vibrational modes originate from the top BZT-BCT layer. Also,

in the sample BZT-BCT/LSMO/LAO, the conducting LSMO reflects the exciting laser

beam so that it does not enter the LAO substrate. Actually, this kind of heterostructure

has several advantages. First, the conducting layer doesn’t allow the laser to enter into the

substrate. Therefore, the Raman signal is exempt of the substrate. Second, the conducting

oxide bilayer ensures the high quality epitaxial growth of the ferroelectric thin film in

comparison to Si, sapphire substrates. Furthermore, such bilayer structures are similar to

39

those used in tunable microwave devices which make the experimental results more

relevant to practical applications.

Both the thin films were grown following exactly the same deposition parameters

and both the Raman spectra denote vibrational modes of BZT-BCT. However, we found

red shifts in each of the Raman peaks for the sample grown on Pt substrate compared to

LAO substrate. These shifts caused by the substrate can come from several sources

including surface charge, surface adhesion, defects, chemical environments and/or strain.

As the same techniques were applied throughout deposition, the deviation of chemical

composition can be ruled out. In addition, the surface charge and surface adhesion can be

canceled due to the same oxide layer on both substrates. Therefore, the epitaxial strain

originating from the substrate is responsible for the shifting of Raman modes. The BZT-

BCT film on LAO substrate was highly oriented along (00l) crystallographic plane

whereas on Pt Substrate it was polycrystalline. The polycrystalline film is fully

constrained by the Pt substrate, resulting in the large in-plane compressive strain. On the

other hand, the (00l)-epitaxial film is almost fully relaxed. Table 5 shows the vibrational

modes of BZT-BCT thin films grown on Pt and LAO substrates.

The high-quality of the BZT-BCT thin films are confirmed by the well-defined

Raman spectra. Taken together, Raman and XRD indicate the presence of non-

centrosymmetric regions that result from the local off-centering of the titanium

(zirconium) atoms.

PLD-grown LSMO thin film was highly epitaxial stimulated by monocrystalline

LAO substrate at higher deposition temperature. Figure 13(a) (red) shows the M-H curve

40

of LSMO/LAO thin film with the field applied parallel to the film plane at room

temperature. Figure 13(b) shows the low field fragments of the hysteresis loops of Figure

13(a). The magnetization was calculated after subtraction of a diamagnetic background

from the substrate. The magnetization increased with increasing magnetic field until the

saturation was reached at 500 Oe. A well-behaved M-H curve was observed for the


in Raman spectroscopy of BZT-BCT/LSMO thin films on Pt and LAO substrates

Peak Position

(cm-1)

Amount of Peak

shift

Vibration mode

Pt Substrate LAO Substrate (cm-1)

169.33 179.94 10.61 A1(TO)

290.04 306.68 16.64 E(TO)/B1

524.16 528.30 4.14 A1(TO)

732.15 742.05 9.9 A1 (LO)/E(LO)

-400.0 -200.0 0.0 200.0 400.0-1.0

-0.5

0.0

0.5

1.0

Ma

gn

etiz

ati

on

(n

orm

ali

zed

)

Magnetic Field, H(Oe)

LSMO/LAO

LSMO/Pt

T = 300K

(a)

-10 0 10 20

-0.2

0.0

0.2

Ma

gn

etiz

ati

on

(n

orm

ali

zed

)


LSMO/LAO

LSMO/Pt

T = 300K

(b)

Figure 13. (a) Magnetization-Applied field (M-H) hysteresis loop of the LSMO/LAO

(red), LSMO/Pt (purple) obtained at room temperature and (b) the low field fragments

of the hysteresis loops of Figure (a)

41

LSMO thin film, which is the indication of ferromagnetism in the sample. The film

exhibited a coercive field of 15.2 Oe and a squareness ratio of 0.34.

On the other hand, LSMO thin film on Pt substrate was randomly oriented. Figure

13(a) (purple) shows the M-H curve of LSMO/Pt thin film. The magnetization increases

with increasing magnetic field until the saturation is reached at a high field of 1000 Oe. A

well-behaved M-H curve indicates the ferromagnetism in the sample. The film exhibits a

coercive field of 11.9 Oe and a squareness ratio of 0.2.

The LSMO thin film on LAO substrate is superior to the film on Pt substrate for

different applications. Both the coercive field and the squareness ratio are much larger for

LSMO/LAO than LSMO/Pt. In addition, a high saturation field for LSMO/Pt indicated

that the sample is magnetically hard than LSMO/LAO.

Figure 14(a) (red) shows the M-H curve of BZT-BCT/LSMO/LAO thin films

with the field applied parallel to the film plane at room temperature. Figure 14(b) shows

the low field fragments of the hysteresis loops of Figure 14(a). The magnetization

increased with increasing magnetic field until the saturation was reached at 500 Oe. A

well-behaved M-H curve indicates the ferromagnetism in the sample. The film exhibits a

coercive field of 4.45 Oe and the squareness ratio of 0.08.

On the other hand, Figure 14(a) (green) shows the M-H curve of BZT-BCT/

LSMO/Pt thin film with the field applied parallel to the film plane at room temperature.

The magnetization increases with increasing magnetic field until the saturation is reached

at a high field of 5000 Oe. The film exhibits a coercive field of 2.51 Oe and the

squareness ratio of 0.075.

42

The coercive field is much larger for BZT-BCT/LSMO/LAO than BZT-

BCT/LSMO/Pt. However, the squareness ratios are almost same. In addition, a high

saturation field for BZT-BCT/LSMO/Pt indicates that the sample is magnetically hard

than the other sample. Table 6 summarizes values from SQUID measurement.

The FMR measurements were carried out using a microwave cavity at a wide

range of frequencies with a static magnetic field applied parallel to the film plane. The

resonance condition in this configuration is given by

-400.0 -200.0 0.0 200.0 400.0

-1.0

-0.5

0.0

0.5

1.0

Ma

gn

etiz

ati

on

(n

orm

ali

zed

)


BZT-BCT/LSMO/LAO

BZT-BCT/LSMO/Pt

T = 300K

(a)

0.0 5.0 10.0

-0.15

-0.10

-0.05

0.00

0.05

0.10

Ma

gn

etiz

ati

on

(n

orm

ali

zed

)


BZT-BCT/LSMO/LAO

BZT-BCT/LSMO/Pt

T = 300K

(b)

Figure 14. (a) Magnetization-Applied field (M-H) hysteresis loop of the BZT-BCT/

LSMO/LAO (red), BZT-BCT/LSMO/Pt (green) obtained at room temperature and (b)

the low field fragments of the hysteresis loops of Figure (a)

Table 6. Static Magnetic properties derived from SQUID measurement for all the

samples

Sample: LSMO/LAO LSMO/Pt BZT-BCT/

LSMO/

LAO

BZT-BCT/

LSMO/Pt

Coercive field, Hc (Oe) 15.19 11.90 4.45 2.51

Squareness Ratio, Mr/Ms 0.341 0.199 0.08 0.075

Saturation Field, Ms(Oe) 500 1000 500 5000

43

𝑓 = 𝛾√(𝐻𝑟 + 𝐻𝑘)(𝐻𝑟 + 𝐻𝑘 + 4𝜋𝑀𝑠) ---------------------(1)

where f is the FMR frequency, γ is the gyromagnetic ratio, Hk is the in-plane uniaxial

anisotropy field, 4πMs is the saturation magnetization, and Hr is the resonance magnetic

field. The other important parameter is ‘FMR linewidth’, which is a sensitive probe for

providing magnetic homogeneity in the thin film. The temperature independent and

narrower linewidths are the signature of the homogeneous sample.

Figure 15(a) shows the experimental FMR absorption derivative versus field

profiles at integral frequencies between 7 and 9 GHz for LSMO/LAO thin film. It is

remarkable to note that the film displays pronounced ferromagnetic resonance at room

temperature. The FMR signal is nearly symmetric but there is a little bit distortion at the

field far away from resonance field on both sides. Though the absorption mode strength

was strongest near 7 GHz, the intensity does not vary so much at other frequencies. That

means the magnetization of the sample does not affect by the microwave excitation. The

horizontal line is the zero-mark and the resonant field (Hr) was calculated from the first

zero-crossing of the differential power absorption before going minimum. The resonant

fields were 1163 to 1730 Oe at different frequencies depending on the excitation source.

It shifts towards higher values as the excitation frequency increases. The linewidths (ΔH)

were calculated from the maximum and minimum of the absorption derivative and the

values were 180 to 200 Oe. Figure 15(b) shows frequency dependent in-plane linewidth

of LSMO/LAO sample. The curve shows a linear fit of the LLG approach25 described by

the following equation:

∆𝐻(𝑓) = ∆𝐻0 +2𝛼

𝛾√3𝑓 (2)

44

where ΔH(f) is the linewidth at frequency f, ΔH0 is the linewidth offset at zero frequency

caused by microstructural inhomogeneities and α is the Gilbert damping parameter. The

linear fit yields a linewidth offset of 34.86 Oe, which is lower than the recently reported

value (541.45 Oe for LSMO/STO). The small value of ΔH0 confirmed the homogeneity

of the sample. Figure 15(c) shows the frequency dependence of FMR resonant field for a

wide range of frequencies. The curve was fitted to the Kittel resonance equation. The

slope of the linear fit was 0.003 which demonstrated the value of the gyromagnetic ratio

500 1000 1500 2000 2500

dP

/dH

(a

.u.)

Magnetic Field (Oe)

7GHz

8GHz

9GHz

Sample: LSMO/LAO

Hr

H(a)

4 6 8 10 12 14 16

100

150

200

250

300

350

Sample: LSMO/LAO

Lin

ewid

th,

H (

Oe)

Frequency (GHz)

Linewidth, H (Oe)

Linear fit

(b)

1200 1400 1600 1800

7.0

7.5

8.0

8.5

9.0 Experimental data

Linear Fit

Sample: LSMO/LAO

Fre

qu

ency

(G

Hz)

Resonance Field, Hr(Oe)

(c)


LSMO/LAO at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow shows the


dependence of FMR linewidth and (c) FMR resonance field for the same sample.

45

of the magnetic thin film. The Gilbert damping parameter was calculated 0.037 and the

in-plane uniaxial anisotropy field was found 1099.7 Oe.


profiles at integral frequencies between 8 and 9.5 GHz for LSMO/Pt thin film. The film

displays a large ferromagnetic resonance at room temperature. The FMR signal at higher

frequencies showed some distortion. The resonant fields were 1500 to 2000 Oe at

different frequencies depending on the excitation source. The linewidths (ΔH) of the

absorption derivative were between 250 to 300 Oe. Figure 16(b) shows frequency

dependent in-plane linewidth of LSMO/Pt sample. The curve shows a linear fit of the

LLG approach described by equation (2) which yields a linewidth offset of 187.09 Oe.

The small value of ΔH0 confirmed the homogeneity of the sample.

Figure 16(c) shows the frequency dependence of FMR resonant field for a wide

range of frequencies. The curve was fitted to the Kittel resonance equation, which yields

a value of 0.0038 for the gyromagnetic ratio. The Gilbert damping parameter was

calculated 0.05 and the in-plane uniaxial anisotropy field was found 126.6 Oe.


profiles at integral frequencies between 7 and 9 GHz for BZT-BCT/LSMO/LAO thin

film. The film displays a large ferromagnetic resonance at room temperature but shows

some distortion far away from resonance. The resonant fields were 1300 to 2000 Oe at

different frequencies. The linewidths of the absorption derivative were between 115 to

145 Oe. Figure 17(b) shows the frequency dependent in-plane linewidth of BZT-BCT/

LSMO/LAO sample. The curve shows a linear fit of the LLG approach described by

equation (2) which yields an offset of 48.17 Oe. The small value of ΔH0 confirmed the

46

homogeneity of the sample. Figure 17(c) shows the frequency dependence of FMR

resonant field for a wide range of frequencies. The curve was fitted to the Kittel

resonance equation, which yields a value of 0.00169 for the gyromagnetic ratio. The

Gilbert damping was calculated 0.02883 and the uniaxial anisotropy field was found

347.1 Oe.


profiles at integral frequencies between 7 and 9 GHz for BZT-BCT/LSMO/Pt thin film.

1250 1500 1750 2000 2250 2500

dP

/dH

(a

.u.)

Magnetic Field (Oe)

8 GHz

9 GHz

9.5 GHz

Sample:LSMO/Pt

(a)

6 7 8 9 10250

260

270

280

290

300

Sample: LSMO/Pt

Linewidth, H (Oe)

Linear fit

Lin

ewid

th,

H (

Oe)

Frequency (GHz)

(b)

1500 1600 1700 1800 1900 2000

8.0

8.4

8.8

9.2

9.6 Experimental data

Linear Fit

Sample: LSMO/Pt

Fre

qu

ency

(G

Hz)


(c)


LSMO/Pt at 7 GHz (black), 8 GHz (red), and 9.5 GHz (blue). The arrow shows the


dependence of FMR linewidth and (c) FMR resonance field for the same sample

47

The film displays some distortion at all frequencies. The resonant fields were 400 to 1800

Oe at different frequencies. The linewidths (ΔH) of the absorption derivative were

between 290 to 360 Oe. Figure 18(b) shows the frequency dependent in-plane linewidth

of BZT-BCT/LSMO/Pt sample. The curve shows a linear fit of the LLG approach

described by equation (2) which yields a linewidth offset of 120.9 Oe. The small value of

ΔH0 confirmed the homogeneity of the sample. Figure 18(c) shows the frequency

dependence of FMR resonant field for a wide range of frequencies. The curve was fitted

500 1000 1500 2000 2500

dP

/dH

(a.u

.)

Magnetic Field (Oe)

7GHz

8GHz

9GHz

Sample:BZT-BCT/LSMO/LAO(a)

6 7 8 9 10100

110

120

130

140

Sample:BZT-BCT/LSMO/LAO

L

inew

idth

,

H (

Oe)

Frequency (GHz)

Linewidth, H (Oe)

Linear Fit

(b)

1200 1500 1800 2100

6

7

8

9

10

Sample:BZT-BCT/LSMO/LAO

Fre

qu

ency

(G

Hz)


Experimental

Linear Fit

(c)


BZT-BCT/LSMO/LAO at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow

shows the resonant frequency and the width (orange line) shows the linewidth. (b)

Frequency dependence of FMR linewidth and (c) FMR resonance field for the same

sample.

48

to the Kittel resonance equation that yields a value of 0.00338 for the gyromagnetic ratio.

The Gilbert damping parameter was calculated 0.0376 and the in-plane uniaxial

anisotropy field was found 23.3 Oe.

For the sample LSMO/LAO the linewidth-offset, Gilbert damping parameter, and

saturation magnetization are quite small compared to LSMO/Pt sample. In addition, a

high squareness ratio and high in-plane anisotropy field for the LSMO/LAO sample

confirmed its superiority than LSMO/Pt.

500 1000 1500 2000 2500

dP

/dH

(a.u

.)

Magnetic Field (Oe)

7GHz

8GHz

9GHz

Sample: BZT-BCT/LSMO/Pt

(a)

6 8 10 12 14 16

300

350

400

450

500


L

inew

idth

,

H (

Oe)

Frequency (GHz)

Experimental data

Linear Fit

(b)

400 800 1200 1600 20005

6

7

8

9

10


Fre

qu

ency

(G

Hz)


Experimental data

Linear Fit

(c)


BZT-BCT/LSMO/Pt at 7 GHz (black), 8 GHz (red), and 9 GHz (green). The arrow

shows the resonant frequency and the width (orange line) shows the linewidth. (b)

Frequency dependence of FMR linewidth and (c) FMR resonance field for the same

sample.

49

For the sample, BZT-BCT/LSMO deposited on LAO substrate, the linewidth-

offset, Gilbert damping parameter, and saturation magnetization is quite small compared

to film on Pt substrate. In addition, the high in-plane anisotropy field for the BZT-

BCT/LSMO/ LAO sample confirmed its superiority than BZT-BCT/LSMO/Pt. However,

the squareness ratio was almost same for both the sample.

The effect of an FE layer capping to the sample LSMO/LAO can be revealed

from the above FMR absorption derivatives. After capping with an FE layer the

linewidth-offset increases a little bit whereas the Gilbert damping parameter, squareness

ratio, and anisotropy field decrease. On the other hand, the saturation magnetization of

LSMO capped with an FE layer increases a lot indicating that a high field is necessary to

reorient the magnetic field. However, after capping with an FE layer all the parameters

except the saturation magnetization decreased. The saturation magnetization increases

almost three times indicating that a high field is necessary to reorient the magnetic field.

All the calculated values from the FMR measurements were presented in Table 7.

Table 7. Dynamic magnetic properties of all the samples calculated from FMR

Sample: LSMO/LAO LSMO/Pt BZT-BCT/

LSMO/LAO

BZT-BCT/

LSMO/ Pt

Linewidth offset, Δ𝐻0 (Oe) 34.86 187.09 48.17 120.9

Gilbert damping parameter, 𝛼 0.037 0.05 0.02883 0.0376

Gyromagnetic ratio,(GHz/Oe) 0.003 0.0038 0.00169 0.00338

Saturation magnetic field, Oe 500 1000 500 5000

Uniaxial anisotropy, 𝐻𝑘 (Oe) 1099.7 126.6 347.1 23.3

50

Conclusions

Both the BZT-BCT and LSMO thin films on LAO were highly oriented

stimulated by the monocrystalline substrate. On the other hand, the pseudo-crystalline Pt

substrate yield randomly oriented thin films. The red shifts of the Raman vibrational

peaks of BZT-BCT originate from the strain of Pt/LSMO interface. The well-behaved M-

H hysteresis loops confirm the ferromagnetic behavior of all the LSMO thin films. The

observed right shift of the hysteresis loops of LSMO in presence of an FE top layer may

be an indication of the desired magnetoelectric coupling between the FE and FM layers.

The advanced applications depend upon this kind of coupling between two different

materials. Thin films on Pt substrate are magnetically hard than the films on LAO

substrate. However, FMR data reveal the dynamic magnetic properties of PLD grown

thin films. Thin films deposited on LAO substrate show the lowest linewidth offset

compared to the films on Pt substrate. The Gilbert damping parameter and the

gyromagnetic ratio are smaller for the thin films on LAO substrate indicating possible

applications mentioned in the earlier sections such as MRAM, data storage, high-speed

switching sensors, actuators etc. Moreover, the in-plane anisotropy fields are much higher

for the films on LAO than the films on Pt indicating strong anisotropy in LSMO/LAO.

On the other hand, the dynamic behavior of LSMO/LAO thin film is quite similar to the

same thin film with the top ferroelectric layer. But, the dynamic properties of LSMO/Pt

change a lot with FE top layer. So, we can conclude that LSMO thin film can be joined

with another ferroelectric layer for device applications when the insulating substrate or

some insulating layer is necessary for isolation. More study on interface behavior and

charge transport mechanism are needed to bolster above statements.

51

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53

CONCLUSIONS

I have successfully deposited heterostructure of ferroelectric BZT-BCT and

ferromagnetic LSMO thin films, which shows the expected magnetoelectric coupling

between them. The BZT-BCT ferroelectric thin film shows polarization switching

behavior with a large remnant polarization of 93.3 μC/cm2, a coercive field of 159.89

kV/cm, and a large dielectric constant with negligible loss. The well-behaved M-H

hysteresis loop at room temperature confirms the ferromagnetic nature of all the LSMO

thin films with high saturation and remnant magnetization. The observed right shift of the

hysteresis loops of LSMO in presence of an FE top layer is an indication of the desired

magnetoelectric coupling between the layers. Moreover, temperature dependent

magnetization confirmed a high Curie temperature of 360K which was very close to the

bulk sample. FMR data demonstrate that the thin films deposited on LAO substrate show

the lowest linewidth offset compared to the films on Pt substrate. The Gilbert damping

parameter and the gyromagnetic ratio are smaller for the thin films on LAO substrate

indicating possible applications mentioned in the earlier sections such as MRAM, data

storage, high-speed switching sensors, actuators etc. Moreover, the in-plane anisotropy

field is much higher for the films on LAO than the films on Pt indicating strong

anisotropy in LSMO/ LAO. On the other hand, the dynamic behavior of LSMO/LAO thin

film is quite similar to the same thin film with the top ferroelectric layer. However, the

dynamic properties of LSMO/Pt change a lot with FE top layer. So, we can conclude that

LSMO thin film can be joined with another ferroelectric layer for device applications

when the insulating substrate or some insulating layer is necessary for isolation.

54

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Documents

Study of Multiferroic Properties of Ferroelectric